For use in superconductors and high speed data networks for information processing and transmission, integrated circuits have been developed in the form of high speed microwave components. Of special significance in these systems arc integrated circuits on GaAs chips, components based upon the GaAs/AlGaAs system, and like systems which have been employed to provide MESFET's and HEMT's, namely, field effect transistors in which the current transport is effected parallel to the surface of the chip.
An important speed-determining parameter, namely, the transit time beneath the gate, is given in such cases by the dimension of the lithographically generated lateral structuring of the gate (&gt;0.2 micrometer).
As early as 1979, a permeable base transistor had been proposed generally in the form of a field effect transistor but having the current flow direction perpendicular to the chip surface so that the transit time under the gate, i.e. the gate length as a result of vertical structuring, was given by the thickness of the epitaxially deposited base layer.
By modern epitaxial methods, such as molecular beam epitaxy (MBE), metal-organic gas phase epitaxy or metal organic chemical vapor deposition (MOCVD) or metal-organic molecular beam epitaxy (MOMBE, CBE, GFMBE), metallic structured bases could be produced with layer thicknesses of the order of several atom layers.
In earlier systems, the metal base was structured with lateral fingers, between which current channels remained open and in which, at the metal-semiconductor junction, a Schottky barrier was formed to provide a depletion zone whose spatial extent could be varied by the bias voltage on the metal base. With a typical space-charge zone extent of about 1000 angstroms in the voltage free state, by appropriate choice of the bias voltage or potential one could provide an opening width of the base of 2000 to 5000 angstroms as controlled by the base bias or potential.
In the past the material from which this type of transistor was fabricated was silicon from which the base was grown in the form of metallic epitaxially grown nickel or cobalt silicides (NiSi.sub.2, CoSi.sub.2).
When GaAs is used as the basic material of the transistor, polycrystalline deposited metal structures were grown over the GaAs epitaxially from the side.
Furthermore, from time to time quasimetallic epitaxially grown ErAs layer structures were discussed as base materials. Breakthrough results have not been attained heretofore at least with a GaAs system.